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Correlation between nutrient conc. and potential vorticity along the isopycnal σ=24.5 kg/m^3 south of the Kuroshio Extension

Fig. 1 shows the scattered plot nutrient concentration (N) versus potential vorticity Q. Various portion of the plots are colored differently in order to indicate potential sources for each quantity (Fig. 2).

There are two interesting portions in Fig. 1. First, N is increasing with Q from low N and moderate Q (magenta) to high N and slightly higher Q (black). Second, Q is increasing from low N and moderate Q (cyan) to low N and high Q (green). I also enlighted the region of very low N and Q (red).

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Figure 1: Scattered plots of nutrient concentration versus potential vorticity along the isopycnal σ=24.5 kg/m^3 between 15°N and 25°N, 150°E and 170°E in 2004.

According to the mean, the south is a source of high N and moderate Q (S1), the center of low N and low Q (S2) and the north as low N and high Q (S3). These sources could be the extremities of the scatter plot of Fig. 1 that thus would indicate a mixing between S1 and S2 and between S2 ans S3 but not between S1 and S3.

In Fig. 2, we see that indeed S1 and S3 are well separated. S2, however, appears all other the place. Notice also that the meridional gradient of N and Q are opposite in signa and in position. Fig. 2 also tells us that the positive correlation between low-to-high N and moderate-to-slightly-higher Q does not follow latitude but is mixed within the southern part of the gyre. What to conclude?

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Figure 2: Scattered plots of nutrient concentration and potential vorticity versus latitude along the isopycnal σ=24.5 kg/m^3 between 15°N and 25°N, 150°E and 170°E in 2004.

Fig.3 is a repeat of Fig. 1 but for a slight different meridional extent and smaller zonal extent. It shows that the tight correlation between N and Q exist only in the center of the gyre, between 15°N and 20°N.

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Figure 3: Same as in Fig. 1 but for points between 10°N and 22°N, 157.5°E and 162.5°E in 2004. Different colors correspond to different latitudinal bands.